Various techniques have been proposed to enhance the reliability of data recorded by combination of multiple storage devices, such as a hard disk drive and a flash memory (see JP-A-2000-207137). For example, a technique called mirroring writes data of identical content into two physically or logically separate storage areas. Even in the event of some failure or trouble occurring in one of the storage areas, the system is continuously operable with the other storage area.
The mirroring technique writing data of the identical content into the two separate storage areas, however, undesirably halves the substantial storage space.